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144

CLAYS

bo regarded as clays proper. These limits are not absolutely fast, passage from
one Xs of'ubstLnce to another being gradual. The content o clay » not
s^to to give sound indications as to the hydraulic properties o the product
tilting from the heating, as these depend also on the method ot heating and
on the condition in which the separate components occur

As wrdH the determination of the accessory components tins g,.ves useful
indi« tŁns when some of then,, such as magnesia, sulphur etc, are present m
suth Entity as to exert a, harmful influence <m the quality ot the cement
products obtained from the materials analysed.

CLAYS

The essential constituents of days are silica and alumina, while ferric
oxide is also habitually present ; as impurities, they may contain greater
or less proportions of calcium and magnesium carbonates, alkali salts,
sulphates, pyrites, and small quantities of manganese oxide and titanic acid.

The tests to be carried out on clays vary according to the uses to be
made of the latter. Tests for clays to be made into ceramic products,
refractory materials, etc., will be omitted, only the chemical analysis of
days for making cements being considered. This analysis includes •

1.  Hygroscopic Water,   -Determined as in limestone.

2.  Loss on Calcination :   Combined  Water,  Carbon  Dioxide........-

The, loss on calc.ina.tion is determined as with limestone and, usually consists
mainly of water.    When carbonates or organic substances are present, in
sensible quantities, the determination of the combined water and carbon
dioxide may be ma.de as with limestone.

3.  Silica.   -About  I gram of the finely powdered, dried substance is
weighed exa.ct.ly, mixed carefully with 4 5 times its weight of dry sodium-
potassium carbonate and heated in a. roomy platinum crucible over a Buiusen
flame, which is kept small at first and is gradually increased later so as to
give a semiTused mass.    The (lame is then maintained for at least half an
hour, the mass being subsequently healed in the blowpipe llame until it is
completely fused, the disaggivgation thus requiring in all about an hour.
When cool, the crucible is placed in a. hi.rge dish (preferably of platinum)
in which it is heated with water on a, wa.ter-ba.th to soften the mass, tlui
erndblc being afterwards removal with careful washing, and dilute hydro-
chloric add gra.dua.lly added until the carbonates are completely decom-
posed.    The liquid is evaporated to dryness on a. water-bath and heated in
an oven at no 115" to render the silica, insoluble, the silica, bring then
treated as in limestone and weighed.    The jtltrtilc serves for the subsequent
determinations indicated in 4.

The. weighed silica is then evaporated in the platinum crucible with a
few drops of sulphuric, acid and about 5 c.c. of hydroUuorie acid, gently
ignited and weighed : the residue, usually only a, few milligrams, consists
of alumina,, ferric, oxide and, maybe, titanic, add, ;vnd its weight is sub-
tracted from that, of the silica, and added to that of the alumina, and ferric
oxide subsequently found,

To estimate the sand separately from the combined silica., 2-5 grams
of the substance are treated with cone, sulphuric add in a platinum crucible*an 80% of clay, the products mayineral Phosphates " (ibid., 1897, p. 663); G. Masoni: " Contribution to^me